The present invention relates to a light emitting device and more particularly, to a safe light emitting device that can avoid fire when either over voltage or over current occurs.
A light emitting diode lamp meets the requirement of energy saving and is widely applied in replace of traditional incandescent lamps. A series light emitting device that includes a plurality of light emitting diode lamps in series used as Christmas light sets is one of the applications of the light emitting diodes.
FIG. 1 depicts a traditional series light emitting device 10 such as a series of decorative lights of a Christmas tree. The light emitting device 10 includes several light emitting diode lamps, i.e. lamps 11 to 14, which are connected in series. Such a light emitting device 10 can be found in U.S. Pat. No. 5,931,570.
The traditional light emitting device 10 is susceptible to either over current or over voltage and may lead to explosion of light emitting diode lamp, if the light emitting diodes break down one by one. The situation could be very dangerous because the explosion and fire would harm people and even damage property. Such shortcomings of the traditional light emitting device 10 mainly come from the characteristics of the power source and the light emitting diode lamps.
The power source from such as a household socket provides a so-called line voltage that has about 15% of fluctuation in voltage and is current-unlimited. This often leads to over voltage and over current.
Furthermore, the characteristic of the light emitting diode lamps worsens the situation. Please refer to FIGS. 2 and 3 for more detailed description of why the characteristic of the light emitting diode lamps would worsen the situation. FIG. 2 depicts the light emitting diode lamp 11 that exemplifies all the light emitting diode lamps 12 to 14 of the traditional light emitting device 10. The light emitting diode lamp 11 generally includes elements of a light emitting diode chip 111, a bonding wire 112 and a lead frame 113. The elements 111, 112 and 113 are encapsulated in a lamp cap 116 made of epoxy material which may explode if over heated abruptly.
FIG. 3 depicts schematically a current to voltage characteristic chart of a light emitting diode lamp. In a normal operation region A, the current of the light emitting diode lamp increases linearly similar to Ohm""s law as the voltage increases. However, in the region B when over voltage occurs, the current jumps dramatically to an unbearable high that increases the temperature of the lamp. If the temperature increasing rate is moderate, the light emitting diode chip will break down. The impedance of the light emitting diode chip will drop down as it broke down. As the light emitting diode break down, the lamp behaves as a conductor, which means the load of the voltage would be added to the unbroken-down LED lamp(s). The added load burdens the unbroken-down LED lamp(s) and even increases the current. Consequently, the other lamp(s) are more susceptible to break down. Accordingly, the driving current in light emitting device 10 will increase step by step when the light emitting diode lamps break down one by one. Finally, the last unbroken-down LED lamp will be subjected to almost all the voltage load of the device 10. The lamp cap of the final LED lamp would explode and cause spark which may result in fire. That would be extremely dangerous.
Therefore, there exists a need to overcome the shortcomings of the traditional series light emitting device.
The present invention provides a safe series light emitting device that can avoid explosion and fire when either over voltage or over current occurs. The light emitting device of the present invention includes a plurality of light emitting diode lamps in series. In the light emitting device, at least one of the light emitting diode lamps is a safe light emitting diode lamp. The safe light emitting diode lamp results in an open circuit when either over current or over voltage in the light emitting device occurs. As a result, the light emitting device would automatically fail before danger of explosion and fire might occur.
Preferably, the safe light emitting diode lamp includes a chip type resistor or similar device that would result in a broken circuit and disconnect the series lamps when either over voltage or over current in the light emitting device occurs. The chip type resistor is preferably disposed between a light emitting diode chip and a lead frame of the safe light emitting diode lamp.
Advantages and spirit of the present invention can be further understood by the following detailed description of the invention and drawings.